Can we change the frame rate for a video to 24 after the image sequence is rendered with a different fps, say 30?
To my knowledge using another frame rate while rendering an image sequence will ,without changing anything else, change the quality of the output video because a different number of the images out of the sequence will be shown in the output video per second. But the other more noticeable effect would be that by packing a different number of frames in each second you would effectively speed up/ slow down the output video.
It is possible to correct for that speed up in the strip options under time, but you run into another issue: While the video is paused it makes no difference in quality, but if the rendered frame rate of the image sequence is not a multiple of the the new frame rate it can lead to glittery motion. This happens because blender sometimes would need frames that lay between the given frames in the images sequence. Because that is not possible it has to decide for one of those frames. The distance between the chosen frames can vary, sometimes be 1, sometimes be 2, so in the rendered video the motion seems to jump.
so most of the time it makes sense to stick to one frame rate from start to finish.
 
For why the frame rate needs to be able to be changed even if you render an Image Sequence:
One simple reason is that the viewport animation works with the same frame rate so to change that you need that option exposed.
Does that answer your questions?
I hope my explanation makes sense.